On both single sensitive and multiple sensitive retractors for seat belts, webbing sensing usually includes one mode wherein a flywheel's inertia is sensed such that the webbing may be locked against withdrawal by acceleration rather than by velocity. (See, for example, U.S. Pat. No. 3,552,676 WEBER, New Zealand Pat. No. 148,694 BRITAX, both typical of such retractors). This "inertial sensing" operates irrespective of the direction of travel of the reel and irrespective of the direction of webbing travel. Therefore locking will be induced when acceleration of the webbing is increased beyond the sensing level, not only as the webbing is withdrawn from the retractor but also when the webbing accelerates beyond the sensing level as it is retracted back onto the reel.
If the webbing during retraction onto the reel suddenly stops, due to a component on the webbing (for example a buckle part) preventing further movement, a high negative acceleration is sensed and the locking mechanism is operated. Inertia in the total system together with the rewind spring load will stretch the webbing or tighten it onto the spool, and there would be a tendency for the reel to rebound to release the belt tension. However since the locking mechanism is actuated before this can happen, the retractor will be locked, and this will prevent the webbing from being withdrawn. There are therefore occasions when the webbing may be retracted onto the retractor spool at such a rate that when all the webbing is on the reel, the retractor locks, and the webbing becomes jammed in such a way that the seat belt becomes unusable unless some force is applied to the webbing to push it back onto the reel and thereby release tension, but this can only happen when the belt is withdrawn by closing its convolutions on the reel, and then relaxed. However this manoeuvre is not normally understood by users, and the seat belt can be regarded as being unusable.
The main object of this invention is to provide means whereby the rate of retraction is such that it is unlikely for the webbing to jam onto the reel.